Field of the Invention
The present disclosure relates to an electrochromic element and to an optical filter, a lens unit, and an image pick-up apparatus each including the electrochromic element.
Description of the Related Art
EC elements that utilize electrochromic (hereinafter sometimes abbreviated to “EC”) materials in which the optical absorption characteristics (absorption wavelength and absorbance) of substances vary through an electrochemical redox reaction have been used in display apparatuses, variable reflectivity mirrors, and variable transmission windows. Among EC materials, organic electrochromic compounds can be designed to have a variable absorption wavelength and can have a high coloring and bleaching contrast. Thus, organic electrochromic compounds are being actively developed.
In such EC elements, one of the greatest problems is to reduce temporal changes in optical characteristics. U.S. Pat. No. 6,188,505 discloses that a non-electrochromic material that is more easily oxidizable than anodic EC materials and a non-electrochromic material that is more easily reducible than cathodic EC materials are used in a complementary EC element, in which an EC material is dissolved in an electrolyte. These materials are hereinafter referred to as “redox buffers”.
In an EC element according to U.S. Pat. No. 6,188,505, an oxidant of a redox buffer is more stable than an oxidant of an anodic EC material, which is a colored body, and a reductant of the redox buffer is more stable than a reductant of a cathodic EC material. Thus, during bleaching action, as long as a charge imbalance is compensated for by the charge amount of the redox buffer, the corresponding oxidant or reductant of the redox buffer is generated, rather than colored bodies in EC materials being left. Because the redox buffer is non-electrochromic, even when the oxidant or reductant is generated, the redox reaction does not affect light transmittance. In other words, the redox buffer functions to provide a charge balance region of invariable color. Thus, in the EC element according to U.S. Pat. No. 6,188,505, a charge imbalance does not directly cause poor bleaching.
However, in U.S. Pat. No. 6,188,505, the redox buffer is more easily oxidizable than anodic EC materials or more easily reducible than cathodic EC materials, and is therefore more reactive than the EC materials in terms of electric potential. Thus, in the general coloring action of the EC element, the redox buffer is more (or at least equally) reactive than the EC materials. Unlike in the absence of the redox buffer, this requires unnecessary electric current not contributing to coloring, increases power consumption, and lowers response speed.
Furthermore, the use of a redox buffer, as in U.S. Pat. No. 6,188,505, does not necessarily correct charge imbalance between display electrodes. More specifically, the use of a redox buffer only decreases the amount of colored body of an EC material (instead, generates an oxidant/reductant of the redox buffer not contributing coloring and bleaching) and does not necessarily have an effect on the charge balance between display electrodes. A charge imbalance in a complementary EC element changes the ratio of a colored body of an anodic EC material to a colored body of a cathodic EC material. More specifically, the coloring ratio of a material having opposite polarity with respect to a material remaining due to charge imbalance becomes smaller than the coloring ratio of the material remaining due to charge imbalance. For example, when an EC element in a charge imbalance state in which a colored body of a cathodic EC material remains is colored, the coloring ratio due to an anode material becomes smaller than the coloring ratio due to a cathode material, as compared with a state of no charge imbalance. Consequently, the actual absorption spectrum deviates from the design absorption spectrum and appears undesirably as discoloration of the absorption color of the EC element. In U.S. Pat. No. 6,188,505, during bleaching action due to charge imbalance, a redox buffer receives the charge of a residual oxidant of an anode material or a residual reductant of a cathode material and thereby suppresses residual coloring of the polarity of one of the anode and the cathode. However, this does not correct charge imbalance between display electrodes and therefore does not correct the shift the ratio of a colored body of an anodic EC material to a colored body of a cathodic EC material. In other words, even a charge imbalance between display electrodes only suppresses coloring in bleaching action, and a charge imbalance during coloring action changes the ratio of coloring in an anode material to coloring in a cathode material.